The proliferation of optical communication networks intended for subscribers has created a strong demand for low-cost and compact optical assemblies. The costs of an optical subassembly (OSA) increases with the number of components that combine to form the subassembly. In order to reduce the number of component parts of an optical assembly, attention has recently been directed to lensless, butt-coupling methods for interfacing an optoelectronic device and an optical waveguide. The precision of alignment that is required between the end of an optical waveguide, and an optoelectronic device varies with application.
For example, on the receiving side of an optical communication system, a received optical signal is opto-electrically converted into an electrical signal by a photodetector such as a photodiode, and information is reproduced according to the electrical signal obtained. Alignment difficulties on the receiver side of an optical communication system may therefore be introduced by characteristics of both the optical waveguide and the photodetector.
The alignment difficulty may generally be addressed by making a detector “artificially” larger than it needs to be, resulting in slower photodetectors with inherently larger rise times, fall times, and settling times. Larger photodetectors may therefore limit system level bandwidth which ultimately limits transmission data rates. The bandwidth of a photodetector is generally determined by the transit time of the photo-generated carriers in the absorption region and the RC time constant. The inherently lower bandwidth, for larger photodetectors, is caused by higher shunt resistance and larger shunt capacitance of the photo conductive areas of the detectors. More rapid response requires a smaller electrostatic capacitance at the depletion layer. The electrostatic capacitance decreases with decreasing depletion region area. Therefore, the diameter of the light receiving portion of high speed photodetectors are typically restricted to minimize the capacitance of the device.
However, optical beams emanating from an optical fiber typically have a relatively wide cross-sectional area that requires a wide depletion region. For example, the divergent beam size of existing fiber to photodetector interface assemblies that do not include focusing elements are typically on the order of about 25 μm, limiting their utility to data rates below about 10 Gbps.
Similarly, optoelectronic transmitters typically transmit a Gaussian beam whose beamwidth increases with distance from the transmitting device. The diameter of the incident beam (i.e. the transmitter output beam incident upon a fiber end face) therefore increases with increasing distance from the optoelectronic transmitter. Thus, the butt coupling efficiency between an optoelectronic transmitter and a slant ended fiber decreases with increasing separation distance between the end face of the fiber core and the optoelectronic device.